Fig. 7: A Interactions between antibiotics Aztreonam (AZE) and Levofloxacin (LEV) for P. aeruginosa³⁵: The first row delineates the profile of the AZE-evolved resistant strain AZE_R, which shows cross-resistance with respect to LEV; and the second row delineates the profile of the LEV-evolved resistant strain LEV_R, which shows collateral sensitivity with respect to AZE.

B Quantifying collateral sensitivity: This two-dimensional concentration space for AZE and LEV, shows susceptibility and resistance regions colored by blue and red surfaces, respectively. The blue arrow represents collateral sensitivity, while the red arrows show cross-resistance. Reflecting on the MIC position, when stressed by antibiotic AZE, the wild-type (WT) strain evolves to variant AZE_R, becoming resistant to both drugs, as the MIC of AZE_R, MIC(AZE_R), exceeds both breakpoints Br_AZE and Br_LEV. However, under LEV antibiotic stress, WT evolves to LEV_R, which remains sensitive to AZE but resistant to LEV, as the MIC of LEV_R, MIC(LEV_R), only exceeds the breakpoint of antibiotic LEV, Br_LEV. This figure, reproduced from ref. ³⁹, illustrates how different phenotypic states connect when a drug is active.